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I have discovered, on YouTube, videos of what is being called a "Desulfating Capacitative Battery Charger".

The components of this device begin with an MFD, 50µF, 370VAC run capacitor, a bridge rectifier, regular 120VAC dial timer, and a voltage meter to monitor voltage at all times.

This device is, as purported by its various designers, a device to desulfate the plates in lead-acid, flooded, AGM, or sealed batteries of various DC voltage configurations.

It appears so far to work as designed, but before I follow any of these instructions to build one, I believe it is missing one vital component, and that is the ability to control maximum voltage output, i.e. 6V, 12V, etc.

Some videos show this connected to a 12VDC car battery, and charging at the rate of 14.73V. I have a battery bank of 6V Trojan Solar batteries valued at $360.00 each new, and I do not trust that this device would not cause damage if the maximum voltage is not controlled.

What would I need to add to such a device to control this maximum voltage output, to prevent over-charging of my 6V batteries, should I decide to build one of these?

Note: While I am not an electronics engineer, I question the use of the descriptor, "capacitative", as I've never heard this word before.

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    \$\begingroup\$ Start by asking your self what do you need to add to the question that might help people answer.... (1) a banana, (2) a portrait of the Mona Lisa or (3) a circuit? \$\endgroup\$
    – Andy aka
    Commented Feb 17, 2018 at 19:17
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    \$\begingroup\$ what you've described so far sounds potentially lethal. \$\endgroup\$ Commented Feb 17, 2018 at 19:30
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    \$\begingroup\$ A link to the video might be great too. \$\endgroup\$
    – vini_i
    Commented Feb 17, 2018 at 19:31
  • \$\begingroup\$ youtube.com/watch?v=N6NluEvBAUU \$\endgroup\$
    – JamesReece
    Commented Feb 17, 2018 at 21:14
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    \$\begingroup\$ This device, if not used correctly, can kill a person, yes. The warnings are that it should never be supplied with 120VAC BEFORE connecting the DC output to the battery. \$\endgroup\$
    – JamesReece
    Commented Feb 17, 2018 at 21:16

3 Answers 3

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The capacitive supply description is correct. This is a capacitive dropping supply. When connected to AC, capacitors look like resistors. The impedance (similar to resistance) depends on the size of the capacitor and the frequency of the AC source. This "resistor" is used to control the current by dropping some voltage across it. In this case, a 50MFD capacitor at 60Hz is about 53 ohms of impedance. The reason they use a capacitor is it's more efficient. The capacitor will dissipate far less heat than the equivalent 53 ohm resistor.

As far as overcharging, there's not a whole lot you can do. This is not meant to be a battery charger outright. This circuit exposes the battery to high voltage pulses to desulfate the plates, not to charge the battery. This is why there is a timer built in. A battery will only take so much abuse and the timer automatically limits that abuse.

If you want to charge batteries then buy a dedicated battery charger. If you want to desulfate batteries then you could reduce the size of the capacitor to reduce the current.

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Capacitive charging uses as “supply” voltage of V_line= 120VAC RMS. As capacitive impedance is:

 Z = 1/(2.π.f.C)

For C=50uF and f=60Hz, Z=53Ω.

Curiously, this “capacitive charger” works almost as constant current power supply, as:

I_charge = (V_line - V_batt)/Z

For 10V < V_batt < 16V (+60% variation), charging current varies 2.08A > i_charge > 1.96A or just -5%.

That is why it could be seen as Constant Current charger.

To answer your question: You could use non-sparking switch to power off the capacitive charger, maybe using TRIAC or Solid State Relay, although a simpler switch based in electromagnetic relay would be easier to monitor voltage battery and cut voltage supply - but DO NOT USE relays close to batteries, especially in closed cabinets, where uncontrolled voltage in batteries results in gassing and shortened battery life.

Note of Caution:

This is a DANGEROUS charger as it is connected to LIVE wiring, where shocks, short-circuits and switch sparking in an environment where there is no protection to overcharging and gassing, could be prone to explosion (sparks in explosive atmosphere). Could be used as a proof of concept, but additional care should be adopted to use it unattended.

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De-sulfating a lead-acid battery with a pulsed current is a legitimate technique, and there are ample choices on the market to do so with relative safety.

This example you show isn’t one of them.

It’s an R-C 'dropping' supply powered directly from the line. Anything connected to it should be considered 'live', that is, also connected to the AC line. These kinds of supplies see use in things like light fixtures and wall switches, where all the electronics are inside a protective housing and only a few mA of current is needed. In that power-limited use this is ok.

On the other hand, this kind of supply is not safe to use for battery charging, not only because of the hazardous voltage, but the lack of adequate care in controlling the high-current pulses can damage the battery or cause an explosion.

Bad news all around. Avoid.

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